Among more than 400 women undergoing treatment with assisted-reproduction technology (ART), the small fraction who reported using cannabis at the time were more than twice as likely to lose a pregnancy than those who had never smoked marijuana, or who had only used it in the past, Dr. Jorge E. Chavarro of Harvard T.H. Chan School of Public Health in Boston and his colleagues found.

Unexpectedly, the small fraction of couples in which the man was the only current marijuana user were significantly more likely to have a baby.

But this finding should be seen as showing lack of evidence for harm, rather than as evidence that pot has a positive effect on male fertility, Chavarro and his colleagues conclude in the journal Human Reproduction.

“The bottom line remains that we know way too little about the reproductive health effects of marijuana,” Chavarro said in an email. “The scarcity of information is particularly concerning given the concurrent trends of expanded legalization, increased perception that marijuana poses no health hazards and increased consumption among men and women of reproductive age, including among pregnant women.”

Just three studies have looked at how marijuana smoking by both partners affects fertility, the researchers note. Two of them, in couples trying to conceive naturally, found no effect. The third, in couples using ART, found no effect on pregnancy or live births but did find users had lower egg yields and fertilization rates.

The new study included 421 women who underwent 730 cycles of ART between 2004 and 2017 at a Boston fertility center. Male partners of 200 of the women also enrolled. Forty-four percent of the women and 61% of the men reported ever using marijuana, while 3% of women and 12% of men admitted to being current marijuana smokers.

During the study, 317 women had a positive pregnancy blood test in a total of 395 ART cycles, including nine women (16 cycles) who were marijuana users. Pregnancy loss occurred in 54% of the marijuana smokers and 26% of the non-users.

Among couples in which the male partner was the only current marijuana user (23 couples, 41 cycles), 48% had a baby, versus 29% of couples in which the man was a non-user.

However, he said, “Most of the human literature to date has been among men on the higher end of use and most show a deleterious effect of marijuana on sperm and testosterone production.”

“The take-home message is still 100% do not use marijuana while pregnant or trying to get pregnant,” said Dr. Nathaniel DeNicola, an assistant professor at The George Washington University in Washington, D.C., who wasn’t involved in the study.

While research is scarce on marijuana use and fertility, he said in a phone interview, 30 studies have looked at marijuana and pregnancy, and the body of evidence shows a “signal” that women who use cannabis in pregnancy are at increased risk of having a low birthweight infant, of delivering pre-term and of stillbirth. “When marijuana is used at least weekly, or more than weekly, that’s when the risk starts to get more concerning,” he said.

In June, a practical guide for physicians published in the journal CMAJ summed up existing evidence on marijuana and fertility, echoing some of these warnings (bit.ly/2MvmZnw). Overall, men's use of cannabis once a week or more was linked to a 29% reduction in total sperm count, and women's use within the past three months was tied to delayed ovulation.

While the evidence does not show that using marijuana affects the ability to conceive for most couples, for those struggling with infertility, marijuana use “could compound their difficulties,” write Dr. Sara Ilnitsky and Dr. Stan Van Uum of the Schulich School of Medicine and Dentistry at Western University in London, Ontario, Canada.